In a process control system that uses pneumatic instrumentation it is very common to use a bellow as a means of providing movement for a feedback control signal. Movement of the bellows must be adjustable to facilitate a range of feedback to allow the user to control the sensitivity of the instrument. A common approach is to use an adjustable valve, proportional valve that will divert or split the feedback signal from the control loop allowing an adjustment of the pressure sent to the bellow by venting the balance to atmosphere. Simply put the control loop feedback pressure to the bellow is regulated by increasing or decreasing exhaust volume through the proportional valve. This design is very effective because it provides a wide range of pressure to the bellow and facilitates very low pressure settings required for the zero movement of the bellows. The challenge of mechanical means of limiting bellow movement is to provide a resistance of the forces introduced by the control loop signal pressure inside the bellow that has a similar range of adjustability to the pneumatic proportional valve. As well, if the design is to be applicable to a variety of applications, it must be universal in design.
In a process control system where a proportion valve is used it is most often set where 60-80 percent of the feedback control loop signal is exhausted to atmosphere allowing a higher level of sensitivity for the instrument. Because of this high exhaust rate many users have to account for this gas loss by using larger gas compressors. In remote locations where the Natural Gas is utilized as an instrument supply there is a significant cost as well as an environmental impact associated with this design.
United States Patent Application 2008/0078449 (Pesek), entitled “Low Consumption Pneumatic Controller,” discloses a pneumatic instrument that has a proportional bellows assembly which has an upper bellows and a lower bellows. The upper bellows is connected to control pressure. The lower bellows is vented to atmosphere. During operation, changes in control pressure cause an expansion or contraction of the upper bellows. The lower bellows provides a counteracting feedback force to counteract control pressure changes and equalize any resulting force differential in the proportional bellows assembly. In order to provide “tuning” or optimization of the proportional bellows response, a cantilever feedback mechanism is provided that provides proportional band adjustment. This proportional band adjustment is based upon a reduction of any minor motion or hysteresis within the proportional bellows assembly. When operating as intended, the cantilever feedback mechanism provides a proportional adjustment in response to minor movement, without exhausting supply fluid to the surrounding atmosphere. In order to improve functioning of such devices there is a need for an improved cantilever feedback mechanism.